The allegation first surfaced in August, roughly 16 years after the woman who would now be a royal grandmother died in a Paris car crash. Officers were tasked with looking into whether there was any truth to it.

"Every reasonable line of enquiry was objectively pursued in order to fully evaluate any potential evidence," police said in a statement released overnight Monday.

"The final conclusion is that whilst there is a possibility the alleged comments in relation to the SAS's involvement in the deaths may have been made, there is no credible evidence to support a theory that such claims had any basis in fact."

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Wildly popular in life and death, Diana died on August 31, 1997, after the car she was riding in slammed into a pillar in a Paris overpass. Her boyfriend, Dodi Fayed, and their driver, Henri Paul, also died.

Investigators concluded that Paul was drunk and speeding when the accident occurred, and despite at least three inquiries -- including a lengthy London police inquiry that poured cold water on all forms of conspiracy theories in Diana's death -- whispers of collusion and cover-up have persisted.

The latest claim, published by Press Association, the Sunday People newspaper and other British media outlets, alleged that members of Britain's elite SAS commando unit were involved in assassinating Diana.

The claim appears to have been sent first to military authorities and then to London police by the parents-in-law of a British special forces sniper after his marriage had fallen apart, according to an article on the website of the Sunday People newspaper. It did not offer a source for its reporting, but the paper indicated that the parents were questioning the integrity of the soldier, who had testified in another soldier's court-martial.

Sunday People said it had seen a seven-page handwritten letter by the in-laws alleging that the soldier, whom the newspaper did not name, had boasted to his wife that the commando unit was behind the deaths.

Neither the Sunday People piece nor an earlier version carried by Press Association offered details of the claimed involvement by soldiers in the deaths.

The princess left behind her two children, Prince William, whose wife recently gave birth to Diana's first grandchild, and Prince Harry. Some 2.5 billion people around the world watched Diana's funeral.